Progetto comune di ricerca

Evoluzione tettono-sedimentaria dei "red beds" cambro-ordoviciani dei domini dell'Atlas e della Meseta (Marocco)

Responsabili di progetto
Alessandro Ellero, Hassan Ouanaimi
MAROCCO - CNRST - Centre National pour la Recherche Scientifique et Technique
CNR/CNRST 2016-2017
Scienze del sistema terra e tecnologie per l'ambiente
Area tematica
Scienze del sistema Terra e tecnologie per l'ambiente
Stato del progetto

Proposta di ricerca

The proposal falls within a long-term active collaboration between Italian and Moroccan geologists, which in the past ten years led to the publication of geological maps of the Anti-Atlas belt and scientific papers. The collaboration has also resulted in two Short Mobility projects by CNR, with the Moroccan partner as the host institution.
Up to now, some of the researches have focused on the Paleozoic sequences of the Anti-Atlas, where the Cambrian-Ordovician passage is well documented. In this project, we propose to study this passage in the areas north of the South Atlas Fault, where the ages proposed for these sediments are indeed rather imprecise, often assigned generically to the Cambro-Ordovician.
The project focuses on the study of the early Paleozoic red beds, until now little explored using modern methodologies and approaches, with the chief aim of constraining age of deposition, source areas and defining structural features. These successions were deposited in basins related to the initiation of the Palaeozoic rifting, that culminated in the opening of the Rheic Ocean and correlative basins separating the Gondwana continent to the south and the continents of Baltica and Avalonia to the north. The rifting began in the Early Cambrian with the break up of a Neoproterozoic passive margin (Murphy et al., 2006; Nance and Linnemann, 2008). At that time, the northern Gondwana margin began to be fragmented giving rise to the development of marine basins associated with voluminous rift-related magmatism.
Despite Paleozoic sequences are well developed in Morocco, in the Meseta and Anti-Atlas belts, data about the timing of rifting, kinematics and collapse-related processes are not yet sufficient to allow the development of well-constrained geodynamic models.
The sedimentary succession related to the Rheic ocean opening were succesively involved in the Variscan orogenic processes, which led to the construction of the structural domains of Mauritanide, Meseta and Anti-Atlas belts. The effects of the subsequent Atlasic (Alpine) orogenic cycle make even more difficult the correlations between the different structural domains. In particular, problems arise for the correlations of the geological formations at the Cambrian-Ordovician transition.
In the Anti-Atlas belt of southern Morocco, Furongian and Tremadocian marine deposits testify the transition from the Cambrian to the Ordovician. North of the South Atlas Fault, ferruginous sandstones and conglomerates (red beds) locally unconformable at the base of the transgressive Floian (Arenigian) deposits, instead characterize the Cambrian-Ordovician transition (Michard et al., 2008).
These red clastic formations occur in scattered outcrops roughly distributed along the main fault zones bordering the crustal blocks of Central and Eastern Meseta. These shear zones are represented by the Western Meseta Shear Zone, separating the Central Meseta from the Coastal Block (areas of Jebilet and Rehamnas), the South Meseta Fault, separating the Meseta from the Anti-Atlas belt (areas of Ait Hkym, Tizi Tichka and Mougueur), and the fault system of the Middle Atlas belt, separating the Central from the Eastern Meseta (areas of Zekkara, Zaian and Tazekka).
The collection and the analysis of the structural data on the outcrops along the main fault zones will allow to explore the possibility whether these faults, (re)activated during Variscan and Alpine (?) tectonics, also controlled the extension, opening and evolution of sedimentary basin hosting the red beds. Temporal constraints on the activity of fault zones will be defined in suitable lithologies whether preliminary petrographic analysis will reveal that they developed in the ductile regime. Under this hypothesis, the study will include: (i) microtextural and microchemical investigations using SEM-EDS and EMP on the main minerals (particularly micas) of carefully selected samples and (ii) geochronological investigation of micas with the 40Ar-39Ar laser method (using the in situ and the step-heating techniques).
From a broad perspective and from a geodynamic point of view, it will be important to gain more information about the role of transcurrent movements in the deformation of the basement and development of the basin.
The age data and the structural and geodynamic characterization will allow to correlate the red beds with similar formations known from further north in Europe, especially in the Iberian and French Armorican massifs (Gutierrez Marco et al., 1990; Ballèvre et al. 2009). These formations can represent outstanding markers for paleogeographic reconstructions of NW Gondwana, enabling to better place Morocco with respect to the opening of the Rheic Ocean and to other fragments of the northern part of Gondwana at the Cambrian-Ordovician transition.
Moreover, the project aims to examine large fault zones at the regional scale both in terms of kinematic and temporal evolution and then verify how these great structures act and are reactivated in time during different orogenic events.

Obiettivi della ricerca

The project has three main objectives:

- age determination of the red beds formation and stratigraphic characterization of the Cambrian-Ordovician transition in the Meseta-High Atlas domain;
- structural analysis of the red beds outcrops finalized to the study of the main fault zones evolution;
- proposal of a geodynamic model of the northern Gondwana margin and paleogeographic reconstruction of the red beds basin, interpreted in the regional context of the northern Gondwana margin.
The project will integrate field data with data obtained through laboratory techniques.
During the field survey, structural analysis of lineaments involved in the tectonic control of the red beds basin will be performed. At the same time, a detailed sedimentological study will be carried out along selected stratigraphic sections, with the collection of rock samples for laboratory analyses.
Laboratory analyses include: i) the study of rock thin sections; ii) dating of sedimentary rocks through paleontological and micropaleontological studies, detrital and 40Ar-39Ar dating of detrital white mica using laser extraction techniques in suitable samples; iii) dating of magmatic events featuring the Cambrian-Ordovician transition, especially in the Moroccan Meseta.

Ultimo aggiornamento: 29/11/2023